An integrated light source integrating a long wavelength band semiconductor light emitting element (hereinafter referred to as a semiconductor laser) and an electric field absorption type light modulator (hereinafter referred to as a light modulator) on a same semiconductor substrate such as an InP substrate is employed as a signal light source for high speed modulation optical communication.
FIGS. 16(a) and 16(b) are a partially broken perspective view showing an entire structure and a cross-section of a main portion along the resonator length direction, respectively, of a structure of an integrated semiconductor laser and light modulator, recited in Journal of Lightwave Technology, Vol. 8, No. 9, 1990, p. 1357-1362.
In these figures, reference numeral 300a designates a light modulator region, reference numeral 300b designates a semiconductor laser region, and reference numeral 300 designates an n type InP substrate. Reference numeral 302 designates an n type InGaAsP light guiding layer, numeral 303 designates an undoped InGaAsP active layer, and numeral 304 designates an undoped InGaAsP buffer layer, and numeral 305 designates a p type InP layer. Further, reference numeral 306 designates an undoped InGaAsP light absorption layer, numeral 307 designates an undoped InGaAsP buffer layer, and numeral 308 designates a p type InP cladding layer. Numeral 310 designates a diffraction grating. Numeral 311 designates a Fe doped InP current blocking layer. Numeral 312 designates an InGaAs contact layer. Numeral 313 designates a p type impurity diffusion layer. Numeral 314 designates a SiN insulating film, and numeral 315 designates a p side electrode for the light modulator. Numeral 316 designates a p side electrode for the semiconductor laser. Numeral 317 designates an n side electrode common to the semiconductor laser and the light modulator.
Process steps for fabricating the integrated semiconductor laser and light modulator shown in FIG. 16 are illustrated in cross-sections in FIGS. 17(a) to 17(d) and in perspective views in FIG. 17(e) to 17(i).
A description is given of the fabricating process. First of all, as shown in FIG. 17(a), a .lambda./4 shifted diffraction grating 310 having 240 nm pitch is formed at a region where a semiconductor laser is to be formed on a (100) plane of an n type InP substrate 301 designated by B in the figure, as shown in FIG. 17(a). An n type InGaAsP light guiding layer 302 having a composition corresponding to a wavelength of 1.3 .mu.m and 0.1 .mu.m thick, an undoped InGaAsP active layer 303 having a composition corresponding to a wavelength of 1.57 .mu.m and 0.1 .mu.m thick, an undoped InGaAsP buffer layer 304 having a composition corresponding to a wavelength of 1.3 .mu.m and 0.1 .mu.m thick, and a p type InP layer 305 about 1 .mu.m thick, are successively grown in this order by epitaxial growth, and a resist film 320 is formed on the p type InP layer 305 (FIG. 17(b)).
A portion of the resist film 320 on a region where a light modulator is to be formed (designated by A in the figure) is removed by a conventional photolithography technique and, thereafter, a dry etching is performed on the p type InP layer 305, the undoped InGaAsP buffer layer 304, the undoped InGaAsP active layer 303, and the n type InGaAsP light guiding layer 302 employing the resist film as a mask, thereby exposing the surface of the substrate 301 at a region where a light modulator is to be formed (FIG. 17(c)).
An undoped InGaAsP light absorption layer 306 having an energy band gap corresponding to a wavelength of 1.44 .mu.m and 0.3 to 0.5 .mu.m thick, an undoped InGaAsP buffer layer 307 having an energy band gap corresponding to a wavelength of 1.25 .mu.m and 0.1 to 0.3 .mu.m thick, and a p type InP cladding layer 308 about 3 .mu.m thick are successively grown by hydride vapor phase epitaxy (hereinafter referred to as VPE), and then a resist film 321 is formed on the p type InP cladding layer 308 (FIG. 17(d)). Next, the resist film 321 is patterned into a stripe shape along the light guiding direction of the semiconductor laser to be formed by conventional photolithography and, thereafter, a dry etching is performed on the semiconductor layers formed on the substrate 301 employing the patterned resist film 321 as a mask, thereby forming the semiconductor layers into a mesa stripe 325 of 2 micron width. Subsequently, the undoped InGaAsP light absorption layer 306, the undoped InGaAsP buffer layer 307, and the p type InP cladding layer 308 grown on the region for the semiconductor laser are removed by etching and, thereafter, an etching groove 326 for electrical isolation is formed between the region for the light modulator and the region for the semiconductor laser, thereby resulting in a state shown in FIG. 7(e).
A high resistance Fe doped InP current blocking layer 311 is grown by VPE (vapor phase epitaxy) so as to bury both sides of the mesa stripe 325 and the electrically isolating groove 326 and, subsequently, an undoped InGaAs contact layer 312 is grown thereon by VPE (FIG. 17(f)).
Thereafter, a dielectric material film 330 is formed on the contact layer 312, and stripe shaped apertures are formed on this dielectric material film 330 at a region where a light modulator is to be formed and a region where a semiconductor laser is to be formed, respectively. Then, a selective diffusion of zinc is carried out employing the dielectric film as a mask, whereby p type diffusion regions 313 are formed at portions of the Fe doped InP current blocking layer 311 and the undoped InGaAsP contact layer 312, which portions are formed on the mesa stripes 325, so that the diffusion fronts of the p type diffusion regions 313 reach the mesa stripes 325 (FIG. 17(e)).
Thereafter, the InGaAs contact layer 312 is selectively etched so that stripe regions of the contact layer 312 remain at the region for the light modulator and the region for the semiconductor laser, respectively, thereby resulting in a state shown in FIG. 17(h).
A silicon nitride film 314 is deposited covering the upper surfaces of the stripe shaped InGaAs contact layer 312 and the Fe doped InP layer 311, and conventional photolithography and etching are employed to form apertures 314a and 314b for forming electrical contact on the silicon nitride film 314 (FIG. 17(i)).
Thereafter, a metal layer for forming a p side electrode is formed on the silicon nitride film 314 burying the apertures 314a and 314b, and this metal layer is patterned to leave portions burying the apertures 314a and 314b and the peripheral parts thereof and, thereafter, a common n side electrode 317 is formed on the rear surface of the substrate 301, resulting in an integrated semiconductor laser and light modulator, which is formed by monolithically integrating a semiconductor laser and a light modulator on the same substrate shown in FIG. 16(a).
A description is given of the operation. In the prior art optical integrated semiconductor laser, the energy band gap of the undoped InGaAsP light absorption layer 306 in the light modulator portion is larger than the energy band gap of the active layer 303 at the semiconductor laser portion, and the light emitted in the active layer 303 at the semiconductor laser portion in the mesa stripe propagates in the undoped InGaAsP light absorption layer 306 in the light modulator portion and is emitted from the cleavage facet of the undoped InGaAsP light absorption layer 306. In this state, when no voltage is applied to the light modulator (in a state of no bias), the light propagating toward the front facet passes through the light absorption layer 306 and out to outside from the cleavage facet of the light absorption layer 306. Then, because the light absorption layer 306 has an energy band gap larger than the energy band gap of the active layer 303, the laser light passes through the light modulator region without being absorbed. On the other hand, when a reverse bias is applied across the light modulator while applying a positive voltage to the n side electrode 317 and a negative voltage to the p side electrode 315, respectively, an electric field is applied to the light absorption layer 306. Then, due to the Franz-Keldysh effect, the energy band gap of the light absorption layer is substantially reduced as shown in FIG. 19, whereby the propagating light is absorbed by the light absorption layer and not taken out from the facet. In this prior art, a reverse bias is applied across the light modulator as described above, so that an optical signal having a transmission characteristic of, for example, about 5 Gb/s is generated.
In the integrated semiconductor laser and light modulator shown in FIG. 16(a), the light absorption layer 306 at the light modulator region and the active layer 303 at the semiconductor region are different semiconductor layers which have different refractive indices and are formed by separate epitaxial growth processes. Additionally, the layers 306, 307, and 308 of the light modulator are grown thick at the junction portion with the semiconductor laser portion during the epitaxial growth and the active layer 303 at the laser diode region and the light absorption layer 306 at the light modulator region are not connected smoothly. Therefore, reflection and scattering of light may occur at the connection portion between the two layers, which results in a deteriorated optical coupling efficiency between the semiconductor laser and the light modulator.
When carrying out a selective growth employing an insulating film, i.e., covering a part of a wafer surface with an insulating film and carrying out a growth only at a region of the wafer surface not covered with the insulating film, a so-called edge growth occurs in which the growth layer becomes thick in the vicinity of the boundary between a portion covered by the insulating film and a portion not covered by the insulating film. Also in a case where crystal growth is carried out on a wafer having a step, an edge growth occurs in which the layer grown on the concave part at the region for the light modulator becomes thick in the vicinity of the step.
The above-described optical coupling efficiency is significantly influenced by the degree of the edge growth, and the degree of the edge growth due to the step of a wafer becomes larger as the step of the wafer becomes larger. In this prior art, the step of the wafer is equal to or larger than the total thickness of the guide layer 302, the active layer 303, the undoped InGaAsP buffer layer 304, and the p type InP layer 305, i.e., 1.3 .mu.m, and the degree of the edge growth is also fairly large.
In addition, the edge growth not only deteriorates the optical coupling efficiency but produces a large step at the surface after the crystal growth, thereby providing a significant obstacle in the process after the ridge formation.
On the other hand, when semiconductor layers are epitaxially grown by MOCVD with a predetermined region of a semiconductor substrate covered by such as a silicon dioxide film or a silicon nitride film, the material gas which is directly supplied to the surface of the semiconductor substrate is thermally resolved on the substrate and is grown thereon as it is, while the material gas supplied to the insulating film does not react on the insulating film, and, is diffused across the insulating film and moves to a portion where the semiconductor substrate is exposed. It is then thermally resolved on the semiconductor substrate and is epitaxially grown thereon. During this epitaxial growth, there arises a variation in the thickness of the semiconductor layer due to a difference in the growth speed of the semiconductor layer between a position close to and a position spaced from the insulating film on the basis of the above-described effect, and the semiconductor layer grown at a position close to the insulating film becomes thick while the semiconductor layer grown at a position spaced from the insulating film becomes thin. In recent years, utilizing the variation in the thickness of a semiconductor layer when an epitaxial growth is performed by MOCVD in a state where an insulating film is formed on a predetermined region on the substrate, it is proposed to produce an integrated semiconductor laser and light modulator by forming semiconductor layers for a semiconductor laser and semiconductor layers for a light modulator simultaneously in the same epitaxial growth.
FIGS. 18(a), 18(b), and 18(c) are diagrams illustrating a structure and a production method of another prior art semiconductor laser and light modulator produced by the above-described method recited in Electronics Letters, 7th Nov. 1991 Vol. 27 No. 23, p. 2138-2140. In FIG. 18(b), reference characters A and B designate an enlarged views illustrating layer structures of the semiconductor layers in the light modulator region and the semiconductor laser region, respectively.
In these figures, reference numeral 350a designates a semiconductor laser region and reference numeral 350b designates a light modulator region. Numeral 351 designates an n type InP substrate, numeral 352 designates an n type InGaAsP light guiding layer, numeral 353 designates an InGaAs/InGaAsP multi-quantum well layer, numeral 355 designates a p type InP cladding layer, and numeral 356 designates a p type InGaAsP cap layer. In addition, numeral 357 designates a diffraction grating, numeral 358 designates a p side electrode for the light modulator, numeral 359 designates a p side electrode for the semiconductor laser, and numeral 360 designates an n side electrode employed commonly for the light modulator and the semiconductor laser.
A description is given of the process for fabricating the integrated semiconductor laser and light modulator. First of all, as shown in FIG. 18(a), a diffraction grating 357 is formed on the InP substrate 351 at a surface of the region for the semiconductor laser and a stripe shaped silicon dioxide film 370 is formed along the light waveguiding direction of the semiconductor laser on both sides of the diffraction grating 357 (in the figure, a region for producing a light modulator portion is closer). The dimension of the silicon oxide film 370 is, for example, about 200 .mu.m.times.400 .mu.m, and the distance between the silicon oxide films 370 (the width of the region where the diffraction grating 357 is formed) is about 200 .mu.m.
An n type InGaAsP light guiding layer 352, an InGaAs/InGaAsP multi-quantum well layer 353, and a p type InP cladding layer 355 are successively epitaxially grown on the substrate 351. Then, in the region between the silicon oxide film 370 (that is a region for a semiconductor laser), because the material elements diffuse on the mask, the growth speed becomes higher than the region where no silicon oxide film is present (a region becoming a light modulator). As a result, the thickness of respective layers at the region where the silicon oxide film is provided becomes about 1.5 times to 2 times as thick as in a region where there is no mask. On the other hand, the layer thickness of the well layer 381b of the MQW layer of the semiconductor laser becomes thicker than the thickness of the well layer 381a of the MQW layer of the light modulator, whereby the energy band gap of the MQW layer of the semiconductor laser becomes larger than the energy band gap of MQW layer of the light modulator (FIG. 18(b)).
Thereafter, a p type InGaAsP cap layer 356 is formed on the p type InP cladding layer 355, and a portion of the InGaAsP cap layer between the semiconductor laser portion and the light modulator portion is removed by etching to separate them. A light modulator p side electrode 358 and a laser diode p side electrode 359 are formed on the separated respective cap layers 356, and a common n side electrode 360 is formed on the rear surface of the substrate 352, thereby completing an integrated semiconductor laser and light modulator monolithically integrated on the same substrate (FIG. 18(c)).
A description is given of the operation. The InGaAs/InGaAsP multi-quantum well layer 353 serves as an active layer at the region of the semiconductor laser and as a light absorption layer at the region of the light modulator. When a forward direction bias is applied across the p side electrode and the n side electrode of the semiconductor laser, carriers are injected into the InGaAs/InGaAsP multi-quantum well layer 353 and a laser oscillation occurs at a wavelength in accordance with the effective energy band gap of the MQW layer and the period of the diffraction grating 357. The energy band gap of the MQW layer depends on the thickness of the well layer of the MQW layer, and as the well layer thickness becomes thinner, the energy band gap becomes larger. As already described, during the selective growth by MOCVD, the well layer thickness is larger in the semiconductor laser region than in the light modulator region, and the band gap energy Eg1 of the MQW layer in the DFB laser region is larger than the band gap energy Eg2 of that in the light modulator region. When the light modulator is set in the no bias state and the DFB laser is set in a forward bias state to continuously oscillate, the laser light of wavelength (.lambda.1=1.24/Eg1) is not absorbed at the light modulator region because Eg1&lt;Eg2 and is emitted from the facet. On the other hand, when a reverse bias is applied to the light modulator, due to the Quantum-Confined Stark Effect of an MQW layer, the absorbing edge due to excitons is shifted toward the long wavelength side as shown in FIG. 20 and the effective energy band gap Eg'2 is shorter than the value at the DFB laser region (Eg'2&lt;Eg1), whereby the laser light is absorbed by the light modulator and the light output is turned off. Accordingly, the laser light can be turned on or off by modulating a voltage applied to the light modulator.
In addition, in an AlGaAs series high output semiconductor laser formed on a GaAs substrate, a lot of surface energy levels are produced at the oscillation facet of the laser. By the influences of the surface energy levels, the vicinity of facet has an equivalent reduction in the energy band gap relative to the laser central portion. Accordingly, the facet vicinity region becomes an absorbing region for the wavelength of the laser light, and the localized heating at the absorbing region increases with an increase in the light output. Since the energy band gap reduces with a temperature rise, absorption of laser light further increases with a temperature rise, presenting positive feedback, thereby finally resulting in melting and destruction. This phenomenon is called as a catastrophic optical damage (hereinafter also referred as COD), being a serious problem in an AlGaAs series high output semiconductor laser. A window structure is formed at a region in the vicinity of the laser oscillation facet as a region having a larger energy band gap than that corresponding to the oscillation wavelength of the laser to reduce the light absorption in the vicinity of the facet, preventing COD.
FIG. 21 is a perspective view illustrating a structure of the vicinity of the laser facet of a high output semiconductor laser having a window structure at the laser oscillation facet, recited in Japanese Journal of Applied Physics, Vol. 30, (1991), 1904-1906. In the figure, reference numeral 401 designates a p type GaAs substrate. Reference numeral 402 designates an n type GaAs current blocking layer, numeral 403 designates a p type Al.sub.0.33 Ga.sub.0.67 As cladding layer, numeral 404 designates a p type Al.sub.0.08 Ga.sub.0.92 As active layer, numeral 405 designates an n type Al.sub.0.33 Ga.sub.0.67 As cladding layer, and numeral 406 designates an n type GaAs contact layer. Reference numeral 407 designates a (101) facet formed by cleavage, and numeral 408 designates an undoped AlGaAs window layer formed on the cleavage facet 407.
A description is given of the fabrication process. First of all, a semiconductor laser structure is produced by a conventional wet etching and an LPE method. In other words, an n type GaAs current blocking layer 402 is epitaxially grown on the p type GaAs substrate 401 and, thereafter, a stripe V-shaped groove reaching the substrate 401 penetrating the current blocking layer 402 is formed at the element central portion by etching. Thereafter, the p type AlGaAs cladding layer 403, the p type AlGaAs active layer 404, the n type GaAs cladding layer 405, and the n type GaAs contact layer 406 are successively grown on the wafer. After the wafer is ground to a desired thickness, it is cleaved into a bar shape of a width corresponding to the resonator length. In a typical high output semiconductor laser, the resonator length is 300 to 600 .mu.m.
A material having an energy band gap larger than that of the laser light is grown on the laser resonator facet 407 produced by cleavage by MOCVD. In this prior art device, the laser oscillation wavelength is 830 nm which corresponds to an energy of about 1.49 eV, and an undoped Al.sub.0.4 Ga.sub.0.6 As layer 408 having an energy band gap of about 1.93 eV is employed as a window layer. After electrodes are formed, coating of the window layer facet and chip separation are performed, thereby completing a laser chip.
In the above-described reference, it is reported that adoption of such a window structure suppresses COD, whereby a high output and a lengthy lifetime are obtained.
In the prior art integrated semiconductor laser and light modulator shown in FIG. 18(c), because the active layer of the semiconductor laser and the active layer of the light modulator, i.e., the light absorption layer, are produced from a continuous semiconductor layer which is formed by the same process, the laser light generated in the semiconductor laser can efficiently propagate toward the light modulator side relative to the prior art integrated semiconductor laser and light modulator shown in FIG. 16(c). In the crystal growth method used in the fabrication of the semiconductor laser and light modulator shown in FIG. 18(c), a difference in the layer thickness of the grown semiconductor layers is positively produced between a position close to and a position spaced from the insulating film on the semiconductor substrate utilizing a difference in the quantity of material gas contributing to the epitaxial growth A portion having a larger energy band gap and a portion having a smaller energy band gap in the same semiconductor layers are produced but strict growth conditions are required and the method lacks reproducibility. In addition, since a selective growth utilizing an insulating film is employed for the crystal growth for the active layer of the semiconductor laser and the light absorption layer of the light modulator, which determine the actual device characteristics, the crystal quality may not be so good as compared with the crystal quality in the conventional crystal growth employing no selective mask. In other words, in operating the integrated semiconductor laser and light modulator, favorable device reliability and lifetime are not obtained.
In fabricating the prior art window structure semiconductor laser shown in FIG. 21, there was a problem that a complicated process of cleaving the wafer into a bar shape having a width corresponding to the resonator length of a semiconductor laser and thereafter performing an epitaxial growth was required. Furthermore, in the process of performing epitaxial growth onto the cleavage facet, the quality of crystal growth of the grown semiconductor layer (window layer) significantly depends on the cleaved state and lacks reproducibility.